A photocurrent is usually obtained by taking the signal, with dark signal contribution subtracted, multiplied by a radiometric gain (units = Coulombs/signal), divided by the integration time.
See if the following paper is useful. It is for standard course based photo current, power and responsivity calculations of photodetectors. There are a few solved out examples available in this pdf. If you need research based extraction of these parameters, you might have to search more intently through web search engine such as yahoo or google and see if you find papers in-line with your research.
Thank you for your response. Please do recommend all my answers if they address some of your concerns. I am attaching one more paper very eloquently composed and full of explicit device physics based analysis. Although the paper is about organic solar cells whereas your issue is probably with organic photodetectors, the analysis and modeling of this paper should be still usable. The authors of this attached paper have done their modeling by Sentaurus TCAD which you probably are also using and should be benefited.
I am providing you the manual, step-by-step running tutorial of the TCAD and the official source of purchase company Synopsis. I believe you have to pay money for your proprietary use and other on-line source downloading could be harmful for your computer. See if you can download any student or education version of the TCAD software from Synopsis. As always, please recommend my answers if they were useful.